A jump rope.
or
Three:
One to write the light bulb removal program,
one to write the light bulb insertion program, and
one to act as a light bulb administrator to make sure nobody else tries to change the light bulb at the same time.
or
You go on ahead. I'll hang around for a while.
or
A cow walking backwards
or
They turn from notating nuclear spins to notating unclear puns.
or